This invention relates to adhesive compositions and methods wherein the viscosity of the adhesive in a hot melt is reduced so that it can be applied with less energy and better wet out the surface of the substrate.
A hot melt adhesive (HMA) is a thermoplastic polymer system applied in a molten state. The molten adhesives are applied to a substrate and then placed in contact with another substrate. The adhesive cools and solidifies to form a bond between the substrates. Hot melt adhesives are widely used in the packaging industry, for example, to seal and close cartons or to laminate multilayer papers.
A wide variety of thermoplastic base polymers, particularly ethylene-based polymers such as ethylene-unsaturated ester copolymers, including ethylene-vinyl acetate (EVA) and ethylene-n-butyl acrylate (EnBA), have traditionally been used in HMA, but often the adhesive formulation requires substantial use of low molecular weight ingredients such as wax and tackifier to adjust the viscosity and glass transition temperature to useful ranges. Thus, ethylene-unsaturated ester copolymer formulations are typically exemplified by a wax with either a low molecular weight or a low crystallization temperature.
The base polymer selection has largely influenced the flow and mechanical properties: (a) viscosity and rheology characteristics; (b) cohesive strength; (c) flexibility; and (d) adhesive strength. The viscosity of the copolymer has typically been a few orders of magnitude higher than the viscosity of the wax. Low viscosity wax has been used to reduce the high viscosity of the copolymer and resin to ensure efficient mixing. This viscosity reduction is particularly important during the mixing and application stages. In the mixing stage, low viscosity is advantageous because it reduces the energy required to mix the adhesive. In the application stage, a low viscosity HMA is required to pump the molten adhesive from the storage tank to the application area and to ensure proper sprayability and/or surface wetting when applied.
The HMA attributes required for good bonding strength, cohesive strength and low viscosity generally conflict with each other. Generally selecting optimum performance is a matter of trading off one property against another, for example, reducing viscosity decreases adhesion and/or cohesive strength. It is challenging to obtain the desired balance between bonding strength and low viscosity.
The adhesion industry continues to search for adhesive compositions having a balance of properties. Accordingly, there exists a current and long felt need for adhesive compositions that overcome known deficiencies in conventional compositions and have advantageous processing properties.